Traditionally, the chemical nickel-plating process makes use of nickel salts and a hypophosphite, e.g. hexahydrated nickel sulfate and monohydrated sodium hypophosphite. It has already been recommended to replace those two components with a single component, namely nickel hypophosphite. It turns that using this single component leads to an increase in the lifetime of solutions for chemical nickel plating and consequently to decreasing the amount of pollution by nickel; in addition, this new formulation gives rise to the resulting nickel deposits being of better quality, in particular by reducing internal tensions.
Nickel hypophosphite can be produced by causing hypophosphorous acid to react with nickel hydroxide or with nickel carbonate or with some other suitable nickel salt. That purely chemical manufacturing technique suffers from being very expensive.
Proposals have already been made in document EP 693 577 to produce nickel hypophosphite by a method that implements the following steps, namely: putting a nickel anode in contact with a solution of hypophosphite anions; applying electrical current from said anode to a cathode that is itself in electrical contact with said solution so that the nickel of the anode is dissolved into the hypophosphite solution, thereby forming a solution of nickel hypophosphite; and recovering and concentrating said solution of nickel hypophosphite.
The same method can be implemented in an electrodialysis cell having three compartments separated by ion exchange membranes, one for cations and the other for anions. In the receptacle acting as an electrolyzer, the central compartment contains a hyposulfite solution of an alkali metal; it is separated from the compartment containing the nickel anode by an anionic membrane, i.e. a membrane that enables anions to diffuse, but that prevents cations from diffusing; in addition, it is separated from the cathode compartment by a cationic membrane, i.e. a membrane that enables cations to diffuse while preventing anions from diffusing. The solutions contained respectively in the anode compartment (referred to as "anolyte") and in the cathode compartment (referred to as "catholyte") are selected to be conductive so that electricity can pass through these solutions when a voltage is applied. Specifically, the anolytic solution contains hypophosphorous acid, and the catholytic solution contains caustic soda, both at predetermined concentration and pH. Nickel hypophosphite forms in the compartment containing the anolytic solution.
Document U.S. Pat. No. 5,716,512 proposes a method and an installation for manufacturing nickel hypophosphite from a solution of nickel sulfate and a solution of sodium hypophosphite by using the electro-membrane technique. In example 8, as illustrated in FIG. 9 of that document, both electrodes are insoluble, a second cell delivering the anolyte to the first under conditions comparable to those that are obtained in example 1 from a nickel anode that is soluble. In that method, a pH regulator is provided to activate or deactivate the electrodialysis cell to adjust and control the pH, at least of the first electrolyte. Doubtless the purpose of monitoring the pH of the solutions contained in the various compartments of the first cell is to avoid unwanted and harmful precipitation of nickel hydroxide at the interfaces of the various membranes.